Influence of temperature-dependent viscoplastic relaxation and strain-induced martensitic transformation on the fatigue life of 304L stainless steel

• Influence of a single tensile load hold time on the low cycle fatigue life of 304L. • At 300 °C, an improvement in fatigue life exceeding 20 % is experimentally observed. • Fatigue life improvement is attributed to viscoplastic effects during hold time. • Competing mechanisms of martensite formation and viscoplasticity at room temperature. • Secondary cyclic hardening behavior at room temperature is predominantly isotropic. This work investigates the interaction between two competing mechanisms on the fatigue life of 304L stainless steel, martensitic transformation and viscoplastic relaxation, as well as the potential fatigue life enhancement of a single hold time applied prior to cyclic loading. At 300 °C, a tensile load hold time of 15 h applied prior to alternating cyclic loading resulted in an increase in mean fatigue life, exceeding 20 % in the studied low cycle fatigue regime. The observed enhancement is primarily attributed to viscoplastic effects during the hold time, which reduces the maximum stress and fatigue crack growth rate in cyclic loading. At room temperature, the opposite effect was observed. A strain-induced martensitic transformation resulted in a secondary cyclic hardening and a brittle final softening phase. The transformation was enhanced by the hold time, which led to increased brittleness and therefore reduced fatigue life. However, viscoplastic relaxation attenuated the detrimental effect of martensite, as was observed by a 15 % decrease in maximum stress. This study not only demonstrates the positive impact of an extended hold time at elevated temperature on the low cycle fatigue behavior but also analyzes underlying competing mechanisms at room temperature through an in-depth experimental investigation.